Doping effect of Zr and Ti on the critical current density of MgB 2 bulk superconductors prepared under ambient pressure

The doping effect of Ti and Zr on the superconducting properties and the microstructure of sintered MgB ₂ bulk materials has been studied. The transition metal is found to work as a sintering assistant, which significantly enhances the grain connection of the MgB ₂ bulk materials during sintering processing. With this technique, we have fabricated Ti-doped MgB ₂ bulk materials consisting of fine 10 nm scale particles and achieved a high J_c over 1 MA/cm ² in 0 T at 20 K. The large enhancement of J_c is explained by the excellent connection between grains and a high density of pinning centers served by grain boundaries and MgO nanoparticles.